1. Field of the Invention
The present invention relates generally to a method for measuring the concentration of a gas in a liquid and, more particularly, to a method of measuring the hydrogen content in molten aluminum and alloys thereof, without having to undesirably compensate for variations in ambient conditions, such as, for example, humidity. The invention also relates to apparatus for facilitating such measurement.
2. Background Information
Hydrogen gas is frequently formed upon exposure of a molten metal, such as molten aluminum, to moisture, such as atmospheric humidity. Hydrogen is much more soluble in molten aluminum and its alloys than in solid state aluminum. Therefore, when molten aluminum starts to solidify, hydrogen solubility rapidly decreases forcing the expulsion of the extraneous gas content. This phenomenon is commonly known in the art as out-gassing. Out-gassing is responsible for metallurgical problems, such as, for example, blisters, voids, blow holes, and reduced corrosion resistance. Accordingly, minimizing such problems requires accurate knowledge of the hydrogen gas content in the molten metal.
There are several known techniques for testing, or extracting hydrogen from, prepared solid aluminum samples, for example, in a laboratory remote from the aluminum casting shop or foundry. One such quantitative laboratory analysis technique is commonly known in the art as Hot Vacuum Subfusion Extraction, also known as the Ransley Method. However, it is well known in the art that analysis of molten aluminum hydrogen concentration levels through “on-line” measuring, or measuring during the aluminum casting process, provides the easiest, fastest, most reproducible and most suitable, on-cite measurement method for aluminum casting shops and foundries.
Known methods and apparatus for determining hydrogen concentrations in molten aluminum typically employ a measurement probe, immersed in the molten metal, and an analyzer for analyzing measurements taken by the probe and for providing a readout of the hydrogen concentration. See, e.g., U.S. Pat. Nos. 2,861,450 and 4,907,440, which are hereby incorporated herein by reference. Three such systems are commonly known in the industry under the names TELEGAS, developed by Alcoa, Incorporated of Alcoa Center, Pennsylvania, ALSCAN, developed by ABB Incorporated of Quebec, Canada and NOTORP, developed by TYK Refractories Company, of Clairton, Penn.
FIG. 1 illustrates a measurement system 2, such as the ALSCAN system manufactured by ABB Incorporated of 585 Charest Boulevard, East, Suite 300, Quebec, Canada. As shown, the measurement system 2 may be directly attached to a launder 16 in, for example, an aluminum casting shop. The measurement system 2 includes a probe 4 structured to be immersed into the molten aluminum 18 and an analyzer 6 for processing data retrieved by the probe 4. See, e.g., U.S. Pat. No. 4,907,440. As shown, the housing 8 of the analyzer 6 encloses a gas reservoir 10 containing an inert reference or carrier gas, such as, for example, nitrogen or argon, and a pump assembly 12.
In operation, the probe 4 is immersed in the molten aluminum 18. The pump assembly 12 circulates a small volume of the carrier gas through the probe 4. Thus, as discussed in the publication “ALSCAN: A New and Simple Technique for In-Line Analysis of Hydrogen in Aluminum Alloys,” by J. P. Martin et al. of Alcan International Limited of Quebec, Canada, which is hereby expressly incorporated herein by reference, the carrier gas is brought into contact with the molten aluminum 18 and recirculated until an equilibrium level is achieved between the hydrogen gas content in the carrier gas and the monoatomic hydrogen content in the molten aluminum 18. The analyzer 6 processes this data and provides a readout of the hydrogen concentration on the analyzer user interface 14.
It is generally recognized that, for a variety of reasons, differing hydrogen measurement results are obtained by different methods of measurement. For example, it has been discovered that, in measuring hydrogen concentration levels with, for example, the ALSCAN system, a bias or shift in hydrogen concentration rejection limits is required to compensate for variations in ambient conditions, such as, for example, seasonal variations in ambient humidity. Rejection limits represent the acceptable upper and lower hydrogen concentration range limitations for the particular molten aluminum alloy being tested. It has been discovered that, for example, when the ambient humidity level is low, the ALSCAN system measurements have rejection limits that are lower than when the ambient humidity level is high. In other words, the hydrogen level readouts provided on the analyzer user interface 14 vary depending on ambient humidity, thus requiring a bias or shift in the acceptable rejection limits. This adjustment or biasing of the rejection limits must be made on a substantially trial and error basis, resulting in uncertainty as to the accuracy and consistency of the measurement system 2 and the hydrogen concentration readings it provides. This problem has led to a constant search for a more efficient and accurate method and apparatus to measure hydrogen quantities in molten metals.
The NOTORP system is a relatively new measurement system using an electrochemical cell mounted inside a sampling tube or probe, which is immersed in molten aluminum. One side of the cell is exposed to the molten aluminum and the other side is in equilibrium with a carrier gas or reference gas such as, for example, argon. A solid state, proton conducting ceramic element is disposed within the immersion head of the sampling tube or probe. The electrochemical potential measured across the solid state, proton conducting ceramic element is processed by the analyzer, which in turn provides a digital readout on the user interface corresponding to the hydrogen concentration level in the molten metal. It has been discovered that NOTORP is substantially unaffected by factors such as ambient humidity. However, the NOTORP probe is complex and therefore costly to replace. Moreover, many aluminum casting shops and foundries are already using an existing measurement system such as an ALSCAN system. Replacement of the existing measurement system, with, for example, a new NOTORP system would be a costly and inefficient endeavor.
There is a need, therefore, for an improved method and apparatus for measuring gas concentrations levels in molten metals that avoids costly replacement of the measurement system currently being employed, and which is substantially unaffected by variations in ambient conditions, such as, for example, humidity.